给定一个二叉树，检查它是否是镜像对称的。
例如，二叉树 [1,2,2,3,4,4,3] 是对称的。
1
/ \
2 2
/ \ / \
3 4 4 3
但是下面这个 [1,2,2,null,3,null,3] 则不是镜像对称的:
1
/ \
2 2
\ \
3 3
说明:
如果你可以运用递归和迭代两种方法解决这个问题，会很加分。
递归
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def help(self, p, q):
if p == None and q == None: return True
if p and q and p.val == q.val:
return self.help(p.right, q.left) and self.help(p.left, q.right) # 重点，左的右=右的左，左的左=右的右
return False
def isSymmetric(self, root):
if root:
return self.help(root.left, root.right)
return True
循环
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def isSymmetric(self, root):
"""
:type root: TreeNode
:rtype: bool
"""
row = [root]
while row:
ret = map(lambda x: x.val if x else x, row)
if ret != list(reversed(ret)):
return False
row = filter(lambda x: x, row)
row = [kid for item in row for kid in (item.left, item.right)]
return True
